Atmosphere and Ocean Origins of North American Droughts

In this paper we take a new look at the causes of droughts over North America over the past century or so, looking both at short term (e.g. one season or year) and long term (e.g. several years such as in the 1930s and 1950s) droughts. To date much work on N.A. droughts has emphasized the role of sea surface temperature (SST) variations, especially in the tropical Pacific Ocean, in driving droughts. We show that this is correct for the southwest and southern Plains regions and, indeed, much of the drought history of those regions can be explained in terms of SST variability. However, that has been known for a while, so in the paper we decided to emphasize the role of internal atmosphere variability showing that this was responsible for some famous droughts (e.g. 1998) and can also overwhelm the tendency for cold Pacific SST anomalies to generate drought (as in the early 1970s - a drought that did not occur). Further for much of North America the ocean influence is limited (e.g. the midwest - an important agricultural area). This is important in that seasonal to interannual predictability relies on predicting SSTs in advance while internal atmospheric variability cannot be predicted beyond the weather prediction timescale. Hence important aspects of drought onset, evolution and termination are probably not predictable since the ocean influence is either not there (in some regions) or provides a relatively modest nudging (in the southwest for example) that the atmosphere can overcome.

We also show and discuss the fact that the state of tropical Pacific Ocean (cool at the equator in the central and eastern ocean) has been favoring drought conditions in the SW since 1998 and this is part of the reason why we have seen ongoing drought in the region. We also show that the influence of human-induced, greenhouse gas-driven, climate change is increasing drought risk (via a reduction in precipitation) in the southwest (other than California) but that this influence is weak right now c.f. that of natural ocean variability on seasonal to decadal timescales.